anthropic: add optional callback handlers

[clutchski]

This pull request adds the ability to add callbacks to Anthropic LLM calls for observability and other use cases. It also adds the missing callback handler and tests. It follows the same patterns as OpenAI (which is fully implemented).

PR Checklist

• [x] Read the Contributing documentation.
• [x] Read the Code of conduct documentation.
• [x] Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• [x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• [x] Describes the source of new concepts.
• [x] References existing implementations as appropriate.
• [x] Contains test coverage for new functions.
• [x] Passes all golangci-lint checks.

[clutchski]

This is related to https://github.com/tmc/langchaingo/pull/949, but that one doesn't the options / tests / etc. It should close that PR.

[KerryJones]

+1 this would be immediately useful